/Engine/Source/Editor/UnrealEd/Public/AssetThumbnail.h
Returns true if the thumbnail for the specified asset in the specified size is in the stack of thumbnails to render
bool IsInRenderStack ( const [TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< [FAssetThumbnail](API\Editor\UnrealEd\FAssetThumbnail) > & Thumbnail ) const